1. Field of the Invention
The present invention relates to an image sensor and a method of forming an image sensor, and in particular to an image sensor comprising an array of pixels.
2. Discussion of the Related Art
Monolithic image sensors comprise photodiodes and transistors formed in a silicon substrate. More specifically, such image sensors comprise an array of pixels each having a pinned photodiode coupled to a sensing node by a transfer transistor. A charge accumulated by the photodiode during an integration period can be transferred to the sensing node via the transfer transistor.
Reading the voltage stored at the sensing node is performed using read circuitry, generally comprising a source follower transistor, having its gate coupled to the sensing node. Furthermore, a reset transistor is also provided coupled to the sensing node, allowing the voltage of the sensing node to be reset after each read. To reduce the number of components, the read circuitry is often shared by more than one photodiode.
The sensitivity of the image sensor is, to some extent, determined by the active surface area within which photons may be captured and converted into electrical charge. Thus to increase the sensitivity of the image sensor, one option would be to increase the size of the active surface area of each pixel. However, there is a problem in doing so without increasing the overall size of the image sensor.